Method for controlling unconsolidated sand in an oil well with permeable cement

ABSTRACT

The movement of unconsolidated sand from a subterranean hydrocarbon-containing formation into a wellbore during recovery of the formation hydrocarbon fluids is prevented by forming a permeable, acid-susceptible cement barrier between the formation and the wellbore. The permeability of said acid-susceptible cement barrier or pack traversing a producing formation is improved by treatment with a composition comprised of mineral acid having dissolved therein a vinylpyrrolidone polymer, which composition slowly reacts with acid-soluble components accumulated at the pack face and within the cement pack so that the permeability of the cement barrier to the flow of formation hydrocarbon fluids is increased while adequate mechanical strength and proper pore size to prevent the movement of unconsolidated formation sand into the bore-hole is maintained and recovering formation hydrocarbon fluids from the formation through the formed permeable cement barrier.

Tate

[ Oct. 30, 1973 METHOD FOR CONTROLLING UNCONSOLIDATED SAND IN AN OILWELL WITH PERMEABLE CEMENT [75] Inventor: Jack F. Tate, Houston, Tex.

[73] Assignee: Texaco Inc., New York, NY.

[22] Filed: Dec. 20, 1971 [21] Appl. No.: 209,887

[52] US. Cl. 166/281, 166/307 [51] Int. Cl E2lb 43/02, E21b 43/27 [58]Field of Search 166/276, 281, 307, 166/311, 312; 252/855 C [56]References Cited UNITED STATES PATENTS 3,179,171 4/1965 Beale, Jr.252/855 C 3,605,899 9/1971 Tate et al.. 166/281 X 3,550,686 12/1970 Knox166/307 2,204,224 6/1940 Limerick et al.. 166/307 UX 3,434,971 3/1969Atkins 166/307 X 3,336,981 8/1967 Barron et al.. 166/281 3,294,76512/1966 Hort et al 166/307 UX 3,428,121 2/1969 Hamsberger 166/2763,429,373 2/1969 Hamsberger et a1. 166/276 Primary Examiner-Stephen .l.Novosad Attorney-Thomas l-l. Whaley et a1.

[57] ABSTRACT The movement of unconsolidated sand from a subterraneanhydrocarbon-containing formation into a wellbore during recovery of theformation hydrocarbon fluids is prevented by forming a permeable,acidsusceptible cement barrier between the formation and the wellbore.The permeability of said acid-susceptible cement barrier or packtraversing a producing formation is improved by treatment with acomposition comprised of mineral acid having dissolved therein avinylpyrrolidone polymer, which composition slowly reacts withacid-soluble components accumulated at the pack face and within thecement pack so that the permeability of the cement barrier to the flowof formation hydrocarbon fluids is increased while adequate mechanicalstrength and proper pore size to prevent the movement of unconsolidatedformation sand into the bore-hole is maintained and recovering formationhydrocarbon fluids from the formation through the formed permeablecement barrier.

25 Claims, No Drawings METHOD FOR CONTROLLING UNCONSOLIDATED SAND IN ANOIL WELL WITH PERMEABLE CEMENT FIELD OF THE INVENTION This inventionrelates to a method for improving the permeability of acid-susceptiblecement packs set into subterranean hydrocarbon-bearing formations. Theimprovement is accomplished by treatement of said cement pack with acomposition comprised of mineral acid having dissolved therein apolymer, which composition reacts slowly to dissolve acid-solublematerials obstructive to fluid movement through the pack faces, orwithin the packs themselves, thereby facilitating the flow of fluidstherethrough and consequently increasing the recovery of hydrocarbonsfrom the formation.

DESCRIPTION OF THE PRIOR ART The recovery of fluids such as oil and/orgas has been found to be difficult in areas where the subterraneanformation is composed of one or more incompetent layers or zonescontaining unconsolidated sand. These unconsolidated sand particles tendto migrate toward the recovery wellbore during recovery of the formationfluid, and frequently the sand particles block the passageways leadingto the wellbore and/or tend to accumulate in the bottom of the wellbore.Plugging or substantially restricting flow of formation fluid toward thewellbore may so drastically decrease the rate of oil recovery that thewell must be shut in even though large quantities of oil remainunrecovered from the formation. Unconsolidated sand particles may movewith the formation hydrocarbon fluid into the wellbore and be carriedalong with the fluid through the tubing and pumps to the surface. Sincesand particles are very abrasive, this occurrence results in abrasivewear and damage to the tubular goods and mechanical equipment. Whileunconsolidated sands are found in many oil and gas producing areas, theyare most common in offshore and coastal marsh regions of the UnitedStates where production is from the more recent geologic sediments, suchas the Miocene formation. Such formations usually contain someacid-soluble material.

Various types of mechanical equipment have been utilized in attempts toprevent the movement of unconsolidated sand particles toward thewellbore and/or into the production tubing and associated equipment.These include placement of sand screens, filters, perforated liners andso forth. While these devices have been used with some success, sandmigration generally still occurs with plugging the result. Furthermore,these devices tend to interfere with certain types of completions andworkover operations. Recently chemical consolidating techniques havebeen used to achieve consolidation of the formation sands while stillproviding sufficient permeability for recovery of formation fluidstherethrough. These methods generally have consisted of injectingpolymerizable resinous material into the underground formation. Afterthese materials polymerize, a barrier is formed which restricts the flowof unconsolidated formation sand and yet sufficient permeabilitypersists to permit recovery of'the formation fluid therethrough. Thesetechniques have not met with widespread acceptance, becasue of theinherent difficulties of effecting uniform polymerization and evendistribution of these resinous materials sufficient to consolidate theseunconsolidated formations and at the same time achieve adequate anduniform permeability so as to allow the unrestricted flow of formationfluids therethrough. Furthermore the cost of the resinous materials usedfor this purpose is relatively high in comparison with prior mechanicalmethods.

In U. 5. Pat. No. 3,428,121 and U. S. Pat. No. 3,429,373 there aredisclosed methods of forming permeable cement compositions suitable foruse in incompetent formations. These permeable cement compositions areused to form a permeable cement barrier between the unconsolidatedhydrocarbon-containing formation so that the formation fluids can berecovered while the sand particles are restrained from entering thewellbore by the permeable barrier. While these materials and procedureshave been found to be successful, there are instances when it isdesirable to increase further the permeability of the permeable cementbarrier produced in accordance with U. S. Pat. No. 3,428,121 or U. S.Pat. No. 3,429,373. More particularly, it is sometimes desirable in thecase of new cement packs to' use an oil squeeze or acidization treatmentto promote initial flow of formation fluids therethrough. By oil squeezeis meant the common field practice of pumping oil through the cementbarrier to displace mobile particulate matter from the flow channels,thereby increasing the permeability of the cement barrier. Acidizationis the process of pumping mineral acid, usually hydrochloric acid, intothe set cement barrier, to dissolve a portion of the cement, therebyenlarging the flow channels and increasing the permeability of thebarrier. Such treatment may cause the cement matrix to become soweakened that the cement pack, and indeed the formation itself, maycollapse due to excessively rapid erosion by the acid.

SUMMARY OF THE INVENTION By the method of the present invention one isable to treat effectively the permeable cement barrier so as to increaseits permeability while avoiding the disadvantages of these prior artmethods.

One object of the present invention is to provide an improved method forincreasing permeability more uniformly ofa cement pack placed in awellbore drilled into a hydrocarbon-bearing formation to stabilize saidincompetent formation; An additional object is to provide a method forthe acid treatment of a cement pack in a 'wellbore to removeacid-soluble components thereof with a minimum loss in the mechanicalstrength of the cement pack, thus maximizing the flow of desired fluidsand particularly petroleum hydrocarbons therethrough. 1

According to the present method, a treating composition consistingessentially of a mineral acid having dissolved therein avinylpyrrolidone polymer which polymer greatly reduces the rate ofreaction of the mineral acid with the cement barrier, is injected downthe wellbore under pressure and into the set cement barrier. Thistreating composition is allowed to remain in contact with the cementbarrier for sufficient time for the retarted acid to react with theacid-soluble components accumulated at the pack face and within the packitself, so that the flow channels are enlarged and the permeability ofthe barrier to the flow of fluids is thereby increased substantially.The method can also be employed in a two-step procedure whereby a slugof the acid-polymer solution is introduced via a wellbore into thecement pack and which is thereafter followed by a slug of brine or waterto dilute the acid and thus quench the reaction wherein the successionof slugs is forced into the pack under pressure. After the treatment hasbeen carried out by one of the methods cited above, the well isbackflowed and placed on production.

The objective of using this cited treating composition is to achieveuniform controlled reaction of the acid components with the acid-solublecomponents of the cement barrier and with acid-soluble materialaccumulated at the pack face. The use of a mineral acid, such ashydrochloric acid, is frequently not satisfactory for this purposebecause of its rapid reaction rate with the cement pack. Exposure to apercent hydrochloric acid for even a moderate time results in totaldisintegration ofa hardened sand-cement composition. Use of a dilutemineral acid is likewise unsatisfactory because the acid portion thereofis spent in the portion of the cement-containing barrier firstcontacted. The section of a cement barrier nearest the wellbore andtherefore that section first to react with a dilute mineral acid wouldhave a substantially increased permeability and very substantiallydecreased mechanical strength to the point of disintegration. Thesection of the cement barrier fartherest from the wellbore, and so lastto be contacted by the spent dilute mineral acid, would be virtuallyunaffected by the spent acid, and so would have insufficientpermeability for the passage of formation hydrocarbon fluidstherethrough.

One especially attractive and unique feature of the present invention isthe ability to monitor the progress of the treating method and therebyadjust the reaction time of the treating composition to produce thedesired cement barrier permeability. By measuring the pressure and flowrate of the treating composition during the injection thereof into thehardened cement barrier, a good indication of the extent of improvementin permeability of the said cement barrier can be obtained. The treatingcomposition can be left in contact with the cement barrier for whatevertime is required to achieve the desired permeability increase, and thenremoved before excessive reaction and consequent reduction incompressive strength has occurred.

The permeable cement barrier formed in accordance with the teachings U.S. Pat. No. 3,428,121 and U. S. Pat. No. 3,429,373 may be treated withthe treating composition of the present invention to increase thepermeability of the permeable cement barrier immediately aftercompletion of the field procedure described in these above citedpatents, to reduce anomalously low permeability resulting from faultyfield procedure, or the treatment may be carried out some time afterformation of the permeable cement barrier to correct a decrease inpermeability resulting from plugging of the cement barrier by finesuspended material which is deposited against or within the cementbarrier by formation fluid. in either event, the treating compositionand procedures for injecting same into the cement barrier are identical.

In formulating the treating composition of this invention, an aqueoussolution of from 3 to about 30 percent by weight of a non-oxidizingmineral acid such as hydrochloric acid, sulfuric acid, etc., is firstprepared. An inhibitor to prevent corrosion of acid on the metalequipment associated with the well is usually added with mixing in thenext step. The vinylpyrrolidone polymer in an amount within theconcentration range of from 0.1 to about 10 percent by weight based onthe total solution weight is then admixed with the aqueous acid solutionemploying a blender. The polymer dissolves rather rapidly in the acidsolution. The composition is forced via a suitable pumping system, downthe wellbore and into contact with the cement barrier to be treated. Thepressure employed is determined by the permeability of the cementbarrier, the viscosity of the fluid and other operating variables. Theinjection of this treating composition should be carried out at apressure sufficient merely to penetrate the cement barrier. Generally,it is advisable to allow the aqueous acidic polymer solution to remainin contact with the cement barrier for sufficient time to achieve thedesired improvement in permeability or until measurements of flow rateand injection pressures during the injection of the treating compositionindicate the desired cement barrier permeability has been obtained.After this, the substantially spent treating solution is reversed out ofthe well, i.e., it is allowed to flow back out or to be pumped out ofthe cement barrier.

Water-soluble vinylpyrrolidone polymers useful in preparing the novelcompositions of this invention include those having recurring units ofthe formula:

wherein R, R,,, R,, and R are independently selected from the groupconsisting of hydrogen and alkyl radicals having from 1 to 5 inclusivecarbon atoms are employed. Examples of alkyl radicals of l to 5inclusive carbon atoms include methyl, ethyl, propyl, butyl, pentyl andisomeric forms thereof. The average molecular weight of thevinylpyrrolidone polymer utilized in the method of this inventiongenerally will be from about 10,000 to about 1,000,000 or more and,preferably from about 10,000 to about 400,000. It is critical in thepractice of the present invention that the vinylpyrrolidone polymer besoluble in the aqueous acid. When R, R R,, and R of Formula I above areeach hydrogen, the resulting compound is polyvinylpyrrolidone, i.e.,poly-N-vinyl-2-pyrrolidone, which is an especially useful polymer.

Preferably, the acidic polymer solution of this invention is onecomprising an aqueous solution of about 3 to about 30 percent by weightof a non-oxidizing mineral acid, which may or may not include brine, andwhich contains dissolved therein between about 0.1 to about 10 percentby weight based on the total solution weight of the water-solublevinylpyrrolidone polymer.

Generally, the acidic polymer solution will contain an inhibitor toprevent or greatly reduce the corrosive attack of the acid on metal. Anyof a wide variety of compounds known in the art and employed for thispurpose can be used. Compounds of arsenic, nitrogen or sulfur asdescribed by Grebe et al. in U. S. Pat. No. 1,877,504 or a rosin aminetype as described in U. S. Pat. No. 2,758,970 would be suitable. Theamount of the inhibitor utilized is not highly critical and it may bevaried widely. Usually this amount is defined as a small but effectiveamount, e.g., from 0.02 percent to about 2.0 percent by weight or moreof the acidic polymer solution.

The preparation of vinylpyrrolidone polymers suitable for use in thetreating composition of this invention is described in Kirk-Othmer,Encyclopedia of Chemical Technology, Vol. 1, Second Edition,Interscience Publishers, New York, 1963, p. 205. Such vinylpyrrolidonemonomers can be synthesized by reacting a carboxylic acid amide such as2-pyrrolidone, 3-methyl-2-pyrrolidone, 4, 4-diethyl-2-pyrrolidone,5-isobutyl-2-pyrrolidone, 4-methyl-2-pyrrolidone, etc., of the formula:

0 mo+on l Rb H :0

RC CR wherein R and R,, have the same meaning as previously described,under pressure at a temperature of about 130 to about 160C. and in thepresence of the alkali metal salts of these acetylenic compounds ascatalysts. Polymerization of the resulting monomers can be conducted bymethods well known in the art.

Among the advantages resulting from employing the treating compositionin treating cement barriers to improve the permeability thereof are thefollowing:

1. The reaction rate of the acid with the acid-solubles in the cementbarrier is greatly reduced. A problem encountered in the use of mineralacids for this purpose is the very rapid rate with which known mineralacids react with such acid-solubles so that little actual effect takesplace at any great distance from the point of first contact between themineral acid and the cement barrier. This rapid erosion of acid-solubleswould likely lead to mechanical failure of the cement barrier.

2. The viscosity of the treating composition is increased. This helpspromote uniform penetrating of the cement barrier. I i

The following is a description by way of example of tests demonstratingthe effectiveness of the present invention.

In the first series of tests, a cement composition was prepared bymixing 1,400 grams of 20-40 mesh Ottawa sand, 254 grams ofa light weighthydraulic cement, and 175 ml. of a brine prepared by'dissolving 3 gramsof calcium chloride and 30 grams of sodium chloride in 100 millilitersof water. Cylindrical plugs ,were prepared from this cement compositionand their compressive strengths measured after the plugs had hardenedfor several hours.

The plugs were weighed and mounted in Hassler sleeves so that thetreating composition could be passed therethrough. In each run, the plugwas first water-wet by passing 250 milliliters of distilled watertherethrough, followed by the treating solution. The

partially spent acid solution was collected for subsequent analysis. Afinal volume of water was passed through the plug to remove the acidcompletely. The plug residue was dried overnight and then weighed todetermine the weight loss due to acid dissolution, and the mechanicalcompressive strength was measured. In Run A, 15 percent hydrochloricacid was passed through a plug. In Run B, 15 percent hydrochloric acidhaving dissolved therein 0.5 percent polyvinylpyrrolidone of molecularweight 360,000 was passed through a second plug. In Run C, 15 percenthydrochloric acid having dissolved therein 1.0 percentpolyvinylpyrrolidone of molecular weight 360,000 was passed through athird plug. The results tabulated in Table I were obtained.

1.0% PVP *average compressive strength before treatment was 1,120 psi.

The data in Table I show the superiority of the retarded acid inachieving the desired results. The lower weight loss and lowerconcentration of calcium in the partially spent acid solution in thetests wherein the retardant of the subject invention is used indicate areduced rate of dissolution of acid-soluble components of the cementmatrix. The compressive strength measurements indicate that use ofretarted acid preserves the mechanical integrity of the cement matrix,whereas conventional, unretarded acid completely destroys the cementmatrix.

In a second series of tests the viscosity of solutions of 0.5 percentand 1 percent by weight of the same polyvinylpyrrolidone polymeremployed in the first series of tests dissolved in 15 percent by weightaqueous hydrochloric acid over extended periods of time was mea sured.Many polymeric materials hydrolyze or otherwise decompose in mineralacids over relatively short periods of time.

Accordingly, the viscosity of the above-described polymer solutions inhydrochloric acid was measured over a period of 28 days. The results areset forth in Table II.

TABLE [1 Concentration of Additive Relative Viscosity Days in 15% HCl)Composition Elapsed 0 0.5 1 l5 I ICI 1.00 Solution of Additive in IICl 31.99 3.40 Solution of Additive in HCl 14 1.93 3.32 Solution of Additivein l-ICl 28 1.92 3.28

The results depicted in Table 11 indicate that little, if any,decomposition of the polymer occurs over a period of 28 days.

I claim:

1. In a method of treating an incompetent, subterranean,petroleum-containing formation penetrated by a wellbore to prevent themovement of unconsolidated sand particles from said formation to thewellbore as formation fluids are recovered from said formation, whereina mixture comprised of sand, acid-susceptible cement and water areinjected via the wellbore against said formation and allowed to set soas to form a partially permeable cement barrier to restrain the flow ofsand, the improvement which comprises injecting into the set cementbarrier a treating composition comprising an aqueous mineral acidsolution having dissolved therein a vinylpyrrolidone polymer to retardthe rate of reaction between the cement barrier and the acidic treatingcomposition, so that the permeability of the cement barrier to the flowof fluids is increased.

2. A method as described in claim 1 wherein the said acid component ofthe treating composition is selected from the group consisting ofhydrochloric acid and sulfuric acid.

3. The method of claim 1 wherein the acidic solution has a concentrationof from about 3 to about 30 percent by weight of the acid.

4. The method of claim 1 wherein the vinylpyrrolidone polymer comprisesrecurring units of the formula:

wherein R, R R and R are independently selected from the groupconsisting of hydrogen and alkyl radicals having from 1 to 5 inclusivecarbon atoms.

5. The method of claim 1 wherein the vinylpyrrolidone polymer has amolecular weight of from about 10,000 to about 1,000,000.

6. The method of claim 1 wherein the vinylpyrrolidone polymer has amolecular weight of from about 10,000 to about 400,000.

7. The method of claim 1 wherein the vinylpyrrolidone polymer ispolyvinylpyrrolidone.

8. The method of claim 1 wherein the said vinylpyrrolidone polymer ispresent in a concentration of from about 0.1 to about percent by weightbased on the total solution weight of the vinylpyrrolidone polymeracidsolution.

9. The method of claim 1 wherein the treating composition is maintainedin contact with the cement barrier for a time sufficient for the acid toreact chemically with the acid-soluble components of the cement barrierand to etch passageways therethrough thereby increasing substantiallythe formation fluid flow capacity of the said cement barrier while stillmaintaining the effectiveness of the said cement barrier for restrainingthe flow of unconsolidated sand into the wellbore.

10. The method of claim 1 further comprising the step of removing thetreating composition and its reaction products by reducing injectionpressure and permitting said treating composition to flow back into thewell.

11. The method of claim 1 further comprising the step of washing thecement barrier with salt water prior to injecting theacid-vinylpyrrolidone polymer solution.

12. A method for increasing the permeability of a permeable cementbarrier in a wellbore penetrating an incompetent subterraneanhydrocarbon-containing formation which comprises injecting into saidcement barrier an aqueous solution of mineral acid having dissolvedtherein a vinylpyrrolidone polymer.

13. The method of claim 12 wherein said acid is selected from the groupconsisting of hydrochloric acid and sulfuric acid.

14. The method of claim 12, wherein said acid is hydrochloric acid.

15. The method of claim 12 wherein said acid is sulfuric acid.

16. The method of claim 12 wherein the acid solution has a concentrationof from about 3 to about 30 per-- cent by weight of the acid.

17. The method of claim 12 wherein the vinylpyrrolidone polymercomprises recurring units of the formula:

wherein R, R,,, R, and R are independently selected from the groupconsisting of hydrogen and alkyl radicals having from 1 to 5 inclusivecarbon atoms.

18. The method of claim 12 wherein the vinylpyrrolidone polymer has amolecular weight of from about 10,000 to about 1,000,000.

19. The method of claim 12 wherein the vinylpyrrolidone polymer has amolecular weight of from about 10,000 to about 400,000.

20. The method of claim 12 wherein the vinylpyrrolidone polymer ispolyvinylpyrrolidone.

21. The method of claim 12 wherein the vinylpyrrolidone polymer ispresent in a concentration of from about 0.1 to about 10 percent byweight based on the total solution weight of the vinylpyrrolidonepolymeracid solution.

22. The method of claim 12 wherein the treating composition ismaintained in contact with the cement barrier for a time sufficient forthe acid to react chemically with the acid-soluble components of thecement barrier to etch passageways therethrough thereby increasingsubstantially the formation fluid flow capacity of the said cementbarrier while still maintaining the effectiveness of the said cementbarrier for restraining the flow of unconsolidated sand into thewellbore.

23. The method of claim 12 further comprising the step of removing thetreating composition and its reaction products by reducing injectionpressure and permitting said treating composition to flow back into thewell.

24. The method of claim 12 further comprising the step of washing theface of the permeable cement bar- 9 a 10 rier with salt water beforeinjecting the acidthe treating composition has reacted with the cementvinylpyrrolidone polymer solution. barrier to remove the residualacid-polymer and reac- 25. The method of claim 12 further comprising thetion products.

step of washing the cement barrier with salt water after

2. A method as described in claim 1 wherein the said acid component ofthe treating composition is selected from the group consisting ofhydrochloric acid and sulfuric acid.
 3. The method of claim 1 whereinthe acidic solution has a concentration of from about 3 to about 30percent by weight of the acid.
 4. The method of claim 1 wherein thevinylpyrrolidone polymer comprises recurring units of the formula: 5.The method of claim 1 wherein the vinylpyrrolidone polymer has amolecular weight of from about 10,000 to about 1,000,000.
 6. The methodof claim 1 wherein the vinylpyrrolidone polymer has a molecular weightof from about 10,000 to about 400,000.
 7. The method of claim 1 whereinthe vinylpyrrolidone polymer is polyvinylpyrrolidone.
 8. The method ofclaim 1 wherein the said vinylpyrrolidone polymer is present in aconcentration of from about 0.1 to about 10 percent by weight based onthe total solution weight of the vinylpyrrolidone polymer-acid solution.9. The method of claim 1 wherein the treating composition is maintainedin contact with the cement barrier for a time sufficient for the acid toreact chemically with the acid-soluble components of the cement barrierand to etch passageways therethrough thereby increasing substantiallythe formation fluid flow capacity of the said cement barrier while stillmaintaining the effectiveness of the said cement barrier for restrainingthe flow of unconsolidated sand into the wellbore.
 10. The method ofclaim 1 further comprising the step of removing the treating compositionand its reaction products by reducing injection pressure and permittingsaid treating composition to flow back into the well.
 11. The method ofclaim 1 further comprising the Step of washing the cement barrier withsalt water prior to injecting the acid-vinylpyrrolidone polymersolution.
 12. A method for increasing the permeability of a permeablecement barrier in a wellbore penetrating an incompetent subterraneanhydrocarbon-containing formation which comprises injecting into saidcement barrier an aqueous solution of mineral acid having dissolvedtherein a vinylpyrrolidone polymer.
 13. The method of claim 12 whereinsaid acid is selected from the group consisting of hydrochloric acid andsulfuric acid.
 14. The method of claim 12, wherein said acid ishydrochloric acid.
 15. The method of claim 12 wherein said acid issulfuric acid.
 16. The method of claim 12 wherein the acid solution hasa concentration of from about 3 to about 30 percent by weight of theacid.
 17. The method of claim 12 wherein the vinylpyrrolidone polymercomprises recurring units of the formula:
 18. The method of claim 12wherein the vinylpyrrolidone polymer has a molecular weight of fromabout 10,000 to about 1,000,000.
 19. The method of claim 12 wherein thevinylpyrrolidone polymer has a molecular weight of from about 10,000 toabout 400,000.
 20. The method of claim 12 wherein the vinylpyrrolidonepolymer is polyvinylpyrrolidone.
 21. The method of claim 12 wherein thevinylpyrrolidone polymer is present in a concentration of from about 0.1to about 10 percent by weight based on the total solution weight of thevinylpyrrolidone polymer-acid solution.
 22. The method of claim 12wherein the treating composition is maintained in contact with thecement barrier for a time sufficient for the acid to react chemicallywith the acid-soluble components of the cement barrier to etchpassageways therethrough thereby increasing substantially the formationfluid flow capacity of the said cement barrier while still maintainingthe effectiveness of the said cement barrier for restraining the flow ofunconsolidated sand into the wellbore.
 23. The method of claim 12further comprising the step of removing the treating composition and itsreaction products by reducing injection pressure and permitting saidtreating composition to flow back into the well.
 24. The method of claim12 further comprising the step of washing the face of the permeablecement barrier with salt water before injecting theacid-vinylpyrrolidone polymer solution.
 25. The method of claim 12further comprising the step of washing the cement barrier with saltwater after the treating composition has reacted with the cement barrierto remove the residual acid-polymer and reaction products.